1. Field of the Invention
The invention relates to a process for imaging a ferroelectric printing form as well as to a printing head with which the ferroelectric printing form can be imaged.
2. Description of the Prior Art
Under normal conditions, a printing form made of a ferroelectric material or containing a ferroelectric material can be brought into a stable polarized state in keeping with an image to be subsequently printed by the printing form. This is done by exceeding in the material a critical field strength E.sub.c, known as the coercive field strength. During this polarization process, large regions with uniform electric dipolar moment, the domains, are oriented in the field direction and, due to the orientation of the material, form a counterfield that in large part compensates for the externally applied field in the interior of the material.
The ferroelectric material is polarized, for example, by attaching electrodes to both sides of the printing form and applying an electric voltage between them. In the case of removable electrodes that establish touch contact with the surface of the material, there must always be an air gap or non-conducting dielectric layers between the electrodes and the printing form. It is therefore inevitable that, due to varying permittivity, a large part of the electric voltage drops off in these layers and only a small electric field is built up in the ferroelectric printing form. Nonetheless, the field strengths in the blocking layers are sufficient to produce micro discharges through these layers, by means of which the ferroelectric layer is polarized in accordance with the image.
Another method of polarizing a ferroelectric printing form is to charge the surface with charge carriers. For this purpose, the surface is subjected to ion or electron beams from charge carrier sources. Charging the surface of the ferroelectric material with free charges causes an electric field to build up in the interior of the material. If this electric field exceeds the coercive field strength of the ferroelectric material, then the domains flip around in a spontaneous process (spontaneous polarization) and form a counterfield in the interior of the material to the electric field on the surface of the material. As a result, very thin regions known as double layers are created on both surfaces of the ferroelectric material; in these regions, the total potential resulting from the charge declines and the previously free charges are bound to the surface in stationary manner by the electrostatic forces thereby created. In the polarized state, the ferroelectric material therefore has charges on its surfaces that are bound to the surface in stationary fashion by the extremely high field strength which the dipolar orientation produces in the interior of the ferroelectric material. These charges screen off the internal field, so that an almost field-free space is created toward the outside at some distance. The polarized state remains stable as long as no reversal of the internal field occurs. Polarization in the opposite direction is possible if the existing charges are first neutralized with charge carriers of the opposite polarity and then, as the result of further charging, an opposite field in the interior leads to spontaneous flipping of the domains.
A non-contact printing head to polarize the printing form in keeping with the image consists of charging sources that can be controlled in keeping with the image. Non-contact printing heads of this type are known and used in electrographic printing processes. In these processes, free charges are transferred to non-conducting dielectric printing forms in keeping with the image. The image can subsequently be developed with electrically charged ink particles (toner particles). During printing processes with such dielectric printing forms, the charge is largely given off to the printing stock together with the ink particles. The known dielectric printing form does allow copies to be produced; however, it is only possible to do so as long as charges still remain on the surface of the printing form. Once all charges have been depleted during the printing process--for example, after about 10 to 20 copies have been printed--the printing process is over. The printing form must be imaged again. Another disadvantage of the known dielectric printing form is that the charge image remains stable for only a limited time and can be destroyed by contact with conductive materials or moist air. Because of this disadvantage, the printing head for imaging must also work in a non-contact fashion.
An ionographic printing head is known from the Technical Research Publication S 6000 OEM of Delphax Systems. This printing head works in a non-contact fashion during normal printing, because the production of a vacuum by sealing the gap between the printing form and the printing head, as described above, is not possible and cannot be carried out by differential pumping at acceptable expense. It is not possible to use a printing head seal that rests on the printing form, because a charge pattern applied by the printing head would be rubbed off in known dielectric printing forms. The known printing head images the printing form by means of briefly ignited high-frequency discharges. Like the Corona discharge also used by known printing heads, this form of discharge provides a rather low charge yield of less than 10.sup.-6 C/cm.sup.2. This results in a lengthy imaging time. In normal printing, another disadvantage of the discharge is the short free path of the produced charge carriers. On the short path between their sources and the surface of the printing form, the charge carriers, because of collisions with surrounding gaseous atoms, are controllable by electric or magnetic fields only to a slight extent. Electrostatic or magnetic focussing is thus only conditionally possible. Ionographic printing heads are therefore not suitable for the production of high-quality prints.
From German reference DE 38 35 091 A1, a ferroelectric printing form is known that is imaged by means of a known printing head. The printing head is connected to a conventional control unit and produces a charge image on the printing form in keeping with an image to be printed.